Update the DB migration script, which creates the Cisco N1kv tables,
to include the ondelete='CASCADE' option in network and port
binding tables. This allows proper clean up of network and port
binding entries on respective delete operations, when the tables
are populated via DB migration script.
Closes-Bug: #1294434
Change-Id: I5dad7bd718b75cfd8825b08336e834d64bddd196